I usually avoid buying used hardware unless i know and trust the person i'm buying it from, or i know it hasn't been abused (i'm a bit picky).
I fully understand that. I feel the same way. But to be honest unless there are obvious flaws its usually a good way to save money and get something better at the same time. Putting that aside i would say that if you can afford it the r9 290 is a very good pick or if you want green card instead the 780gtx. IMHO opinion nothing beneath the 780 gtx is worth a look if we are talking nvidia. Just bought a 780 myself and i am very well pleased for the moment.
While im looking forward to "Mantle" and "True Audio" i dont think its wise to make it a reason for purchase until we have seen what it can actually do. And if its turns out that games like BF4 is getting a huuge boost well im already stomping it at 100+ fps anyway so i can live with that.